This invention relates generally to ion and plasma sources. More particularly it pertains to plasma and ion sources that utilize a Hall current in the generation of the electric field that accelerates ions in a neutral plasma, and it further pertains to the performance of such sources and their being able to operate in adverse environments in which poorly conducting or nonconducting coatings are formed or deposited upon the ion sources or particular components thereof.
This invention can find application in industrial processes such as sputter etching, sputter deposition, coating and property enhancement. It can also find application in electric space propulsion.
The acceleration of ions to form energetic beams of ions has been accomplished both electrostatically and electromagnetically. The present invention pertains to sources that utilize electromagnetic acceleration. Such sources have variously been called plasma, electromagnetic, and gridless ion sources. Because the ion beams are dense enough to require the presence of electrons to avoid the disruptive mutual repulsion of the positively charged ions, the ion beams are neutralized plasmas and the ion sources are also called plasma sources.
In ion sources (or thrusters) with electromagnetic acceleration, there is a discharge between an electron-emitting cathode and an anode located either within the discharge region or at a boundary thereof. The accelerating electric field is established by the interaction of the electron current in this discharge with a magnetic field located between the anode and cathode. This interaction generally includes the generation of a Hall current normal to both the magnetic field direction and the applied electric field. For efficient operation of a Hall-current ion source, the Hall current must follow a closed pathxe2x80x94i.e., with no interruptions to this path.
A Hall-current ion source can have a circular discharge region with only an outside boundary, where the ions are generated and accelerated continuously over the circular cross section of this channel. The closed path for the Hall current follows a circular path within this circular cross section. This type of Hall-current ion source, called the end-Hall type, has a generally axial magnetic field shape as shown in U.S. Pat. No. 4,862,032xe2x80x94Kaufman et al., and as described by Kaufman, et al., in Journal of Vacuum Science and Technology A, Vol. 5, No. 4, beginning on page 2081. These publications are incorporated herein by reference.
A Hall-current ion source can also have an annular discharge region with both inner and outer boundaries, where the ions are generated and accelerated only over an annular cross section. The closed path for the Hall current follows a circular path within this annular cross section. This type of Hall-current ion source, called the closed-drift type, usually has a generally radial magnetic field shape as shown in U.S. Pat. No. 5,359,258xe2x80x94Arkhipov, et al., and U.S. Pat. No. 5,763,989xe2x80x94Kaufman, and as described by Zhurin, et al., in Plasma Sources Science and Technology, Vol. 8, beginning on page R1. These publications are also incorporated herein by reference.
The cross sections of the discharge regions are described above as being circular or annular, but it should be noted the cross-sections, and hence the Hall-current paths, may have elongated or xe2x80x9crace-trackxe2x80x9d shapes. Such alternative shapes are described in the references cited. It should also be noted that the magnetic field shape can depend on the desired beam shape. For example, a radially directed ion beam would have a magnetic field generally at right angles to the magnetic field that would be used to generate an axially directed ion beam.
Those skilled in the operation of the end-Hall type of Hall-current ion source, described the aforementioned U.S. Pat. No. 4,862,032xe2x80x94Kaufman et al., are aware that a poorly conducting or nonconducting coating can accumulate on the exposed anode surface of this ion source. The coating can result from the direct deposition of material on the anode during a dielectric deposition application. The coating can also result from the operation of the ion source in the presence of gases such as oxygen or nitrogen, which can form a dielectric coating from conducting materials deposited on the anode, or even from the anode material itself. This coating gradually increases the discharge voltage and often prevents restarting the discharge after a prolonged period of operation in an adverse environment. (If the operation is voltage limited, the current will gradually decrease.) In an extreme case, it can result in a premature termination of operation.
The adverse effects of poorly conducting or nonconducting coatings on the anode have also been observed in a closed-drift type of Hall-current ion source, and are described in U.S. Pat. No. 5,973,447xe2x80x94Mahoney, et al., also incorporated herein by reference. The solution proposed therein is to introduce the working gas through a gap in a cooled anode, so that the electrical contact of the discharge to the anode can be sustained through the gap after exposed surfaces of the anode become coated with dielectric coatings.
The preceding examples illustrate the problems caused by poorly conducting or nonconducting coatings on the anodes of Hall-current ion sources. These problems are most frequently observed in applications in which dielectrics are deposited on substrates, with some of the dielectric material also being deposited on any nearby Hall-current ion sources, particularly on the anodes of such sources. Dielectric anode depositions are also observed in applications where reactive gases are present and such coatings can be formed at the anode surface, in some cases incorporating the anode material into the coating. Dielectric anode coatings can even be observed in space electric propulsion applications where the dielectric is deposited on the anode due to the sputtering of some other component of the ion source, which is called a thruster when it is used for propulsion. The nature of the problems encountered due to these coatings range from changing the operating characteristics to preventing operation.
The accumulation of poorly conducting or nonconducting coatings on the anode during operation in an adverse environment constitutes an inherent limitation of the Hall-current ion sources described above. The most common industrial solution for such coatings is maintenance, i.e., disassembly, cleaning and/or replacement of components. Such maintenance, however, interrupts production and increases costs. In space applications, maintenance is generally not practical. Longer operating times in adverse environments would be desirable, preferably with little or no change in operating characteristics.
In light of the foregoing, it is an overall general object of the invention to provide a Hall-current ion source with improved tolerance to the deposition of a poorly conducting or nonconducting coating on the ion source.
A more specific object of the present invention is to provide a Hall-current ion source in which the deposition of a poorly conducting or nonconducting coating thereon has a reduced effect on operating characteristics.
A further object of the present invention is to provide a Hall-current ion source which has an increased operating time without maintenance when subjected to the deposition of a poorly conducting or nonconducting coating thereon.
Yet another object of the present invention is to provide a Hall-current ion source in which the anode surface is protected against deposition by the geometry of the ion source.
Still another object of the present invention is to provide a Hall-current ion source in which an anode coating is minimized though thermal and/or mechanical effects.
In accordance with one specific embodiment of the present invention, a Hall-current ion source of the end-Hall type has an anode that is contoured with one or more recesses in the electron-collecting surface which have areas that are protected from the deposition of externally generated contamination thereon, as well as one or more protrusions that have higher temperatures than the bulk of the anode, thereby increasing the removal or passivation of coatings during operation by the thermal degradation of the coating and the effects of thermomechanical stresses.
In another specific embodiment, which can be combined with the above embodiment, electrically isolated baffle or baffles are located to protect a substantial fraction of the electron-collecting surface of the anode from the deposition of externally generated contamination thereon.